The role of the Eastern Australian Current in the recruitment of tropical fish species on temperate reefs

Harris, David Alan (2013). The role of the Eastern Australian Current in the recruitment of tropical fish species on temperate reefsPhD Thesis, School of Biological Sciences, The University of Queensland.

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Changes in ocean temperature and acidity as a result of climate change are altering the distribution and abundance of species. There is now overwhelming evidence that species (both terrestrial and marine) are shifting their distribution ranges to higher latitudes as these regions become warmer, mainly through impacts of warmer temperature on the success of colonisation and survivorship. Given that temperature is not the only factor determining the successful expansion of a particular species, some range extensions are occurring into what were previously marginal environments and may ultimately be ephemeral, being dependent on complex biological and environmental interactions. The thesis investigates the influence of key climate change stressors (both direct and indirect) that are known to affect the survivorship of the common damsel fish, Pomacentrus coelestis along a large tropical to temperate gradient on the east coast of Australia. Pomacentrus coelestis is consistently recorded on temperate reefs but appears to be seasonally transient at the higher latitudes. In trying to understand the drivers controlling this pattern, this thesis examined the ability of P. coelestis to respond to changes in temperature through modification of the species' distribution, physiology and survivorship. Further investigation of this species’ distributional contraints and their potential to limit population shifts will assist in understanding the restrictions facing tropical species that appear in temperate waters. To assess the potential southward latitudinal shift of P. coelestis, and the potential for increased survivability and population sustainability, growth, changes in population structure and physiological responses were measured in a series of laboratory and field experiments. I assessed the responses of P. coelestis to changes in temperature and identified controlling factors over potential latitudinal range shifts for this species.

In Chapter 2 an examination of distribution patterns of P. coelestis across a latitudinal gradient revealed variable recruitment rates and abundance with an overall decline at higher latitudes. Recruitment trends both seasonally and spatially revealed little synchrony along the gradient. Intensive sampling of nekton and adult populations at a single site (Moreton Bay) revealed no clear correlation between larval fish supply and recruitment. The lack of a clear pattern suggests that the fluctuation of environmental factors that affect recruitment processes make their predictability tenuous at both temporal and spatial scales.

These results led to the focus of Chapter 3, which assessed the relationship between the thermal tolerance of P. coelestis and its distribution from tropical to temperate conditions. The results of this study revealed that patterns were not explained solely by reduced growth in response to cooler conditions at higher latitudes and hence that this particular physiological constraint must not be the sole variable determining survivorship at higher latitudes. The consistently high seasonal mortality at higher latitudes resulted in a lack of adult representation at these sites and made predictions of a relationship between size and locality uncertain. The study demonstrated that, for at least the initial recruitment phase, similar growth rates occur across the latitudinal gradient but that the wider thermal variability encountered at more temperate sites does impact negatively on size of fish and increases mortality.

The potential implications of larval connectivity and differences in selective pressure along the East coast of Australia were explored in Chapter 4. These studies were initiated by using the hyper-variable control region of the mitochondrial gene (343 bp) to infer gene flow through spatial variation in allele and genotype frequencies. This study revealed remarkably low genetic structuring among the disparate populations of P. coelestis occurring along this sizeable latitudinal gradient. At the same time, these results indicated significant connectivity via extensive gene flow between all localities. Limited genetic variation and a subsequent reduced potential for local adaptation were also supported by the presence of a largely homogeneous phenotypic structure. These observations indicate that connectivity among existing source populations of P. coelestis and populations currently occurring in temperate waters is ongoing and that there exists potential for further biogeographic expansion.

The genetic studies were combined with an exploration of the physiological responses of P. coelestis in response to temperature, which reveals a strong thermal influence on individual performance and survivorship. Respiratory efficiency for this species was optimised at an ambient water temperature of 24°C, suggesting limited thermal tolerances are a major determinant of its current range and for future demographics. Results demonstrate that P. coelestis has a limited capacity to acclimate its respiratory scope to temperature variations and that while range expansion to higher latitudes is very likely probable, range contraction from more tropical waters is also likely to occur. These observations indicate a need to consider the scale of thermal tolerances when assessing potential ocean warming impacts.

In summary, the results of the present thesis have outlined the physiological and population responses of a typical reef fish species, indicating the likely impacts on its distribution and abundance as the world's oceans rapidly warm. The results highlight key factors that currently limit adaptive strategies and distribution of tropical marine fish in response to warming oceans. In trying to understand this distribution, it is very clear from the results that the future distribution of P. coelestis will not be determined by a single factor within its biology. Not surprisingly, future changes in the distribution and abundance of tropical species such as P. coelestis are likely to be determined by a combination of interlinked biological and environmental factors that affect not only an individual’s direct survivorship but also the more general physiological responses (e.g. Metabolic and growth rates within a population).